Methods for fraccing oil and gas wells

ABSTRACT

A method for stimulating or fraccing an oil or a natural gas well by adding a liquefied hydrocarbon such as liquid petroleum gas and liquefied natural gas, a proppant and a diluent such as carbon dioxide, nitrogen or mixtures thereof to the well. The order of addition is typically liquefied hydrocarbon then diluent but this order can be reversed and in other circumstances the liquefied hydrocarbon and diluent can be mixed together and fed to the well as a mixture.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from provisional patentapplication Ser. No. 61/776,956 filed Mar. 12, 2013.

BACKGROUND OF THE INVENTION

In the production of natural gas from shale or other “tight-gas”formations, hydraulic fracturing (or “frac” or “fraccing”) is used tobreak up the rock around the well bore and reduce the resistance to gasflow. The frac technique generally requires injecting into the welllarge amounts of fluids that are compressible like nitrogen or carbondioxide or incompressible such as water or liquefied petroleum gas. Thefluids are pumped to high pressure to create large compressive forcesaround the well bore. These forces break the rock and create tinyfissures for gas flow.

The stimulation of natural gas containing formations, such as shale gashas been the subject of intensive study. Several methods of fracturingthese deposits are known and incompressible fluids such as water withother chemicals and solids; e.g., mineral acids and proppants areemployed on the order of 1 to 2% by volume to the injected fluid. Theuse of water results in the generation of a large amount of waste thatis laden with chemicals and creates an expensive disposal issue.Thousands of tons of fluid may be injected during each frac job and muchof this fluid is returned to the surface when the flow is reversed(hereafter called “produced fluids”) and natural gas is produced fromthe well. Alternatively, high pressure frac fluid sources includingcarbon dioxide and nitrogen help overcome the liquid waste disposal costassociated with hydraulic fracturing. However, the frac gas generatedupon drilling has a high initial content of carbon dioxide and nitrogen.This mixture can be difficult and expensive to separate and inevitablyleads to natural gas losses and even venting of the initial dilutednatural gas to the atmosphere because it cannot be readily flared.

Recently GASFRAC has tested the use of liquid petroleum gas (LPG) whichhas high propane content and lowers the amount of frac fluid necessaryas well as being easier to separate from the gas that results fromdrilling. The LPG is readily recoverable and can be reused at laterstages in the overall process. However, injection of LPG at highpressures carries some risk involving explosions. The use of otherliquid hydrocarbons such as gelled hydrocarbons is also possible andwould facilitate the complete use of the resulting well gas; howeverflammability concerns could limit its use.

Hydraulic fracturing is used to produce gas and oil from lowpermeability formations, such as for example, unconventional natural gasreservoirs. Most fracturing treatments use water and polymers with agelling agent as a fracturing fluid.

A hydraulic fracturing process can be energized by the addition of acompressible, sometimes soluble, gas phase into the treatment fluid.When the well is produced, the energized fluid expands and gas comes outof solution. Energizing the fluid creates high gas saturation in theinvaded zone, thereby facilitating gas flowback.

During a flowback period, a mixture of gas is used as the energizedsolution (e.g., N₂ and/or CO₂) and natural gas comes out. Before sellingthe natural gas to a pipeline operator, the concentration of energizinggas needs to be dropped to acceptable levels dictated by the pipelineoperator (usually 2 to 3%). Although the concentration of energizing gasdecreases quite rapidly in the beginning of the process, achieving therequired levels may take considerable time (up to 30 to 45 days).

In order to address the issue of environmental pollution from flaringduring a flowback period, various techniques have to be employed tofacilitate natural gas clean-up by separating it from the energizinggas. Such techniques may include N₂ and/or CO₂ separation by usingmembrane and/or adsorbent (PSA/VSA) technologies, which involves thedeployment and operation of relevant equipment, which in turn adds costto the natural gas production process.

Alternative technologies for shale gas development utilize, for example,liquefied petroleum gas (LPG) in place of conventional fracturingfluids, and specifically, in place of high volume, high pressure slickwater-based fracturing fluids. The unique properties of the LPGfracturing process result in significant savings on material expenses,increased well productivity and fracture as well as flow-back mixtureclean up.

The gelled LPG used in the fracturing process has the ability togenerate the necessary fracture system, carry the proppant through thewellbore and place into the oil and gas reservoir being stimulated. TheLPG used in the process is highly soluble in well formationhydrocarbons. As a result, the LPG process results in less damage toformations than conventional hydraulic fracturing processes. Unlikeconventional treatments where as much as 50% of the carrier remains inthe reservoir and hinders well performance, virtually 100% of the LPGcan be recovered, The obvious advantage of LPG fracturing is that gelledpropane would replace the use of water, thereby reducing the amount offresh water used and the associated environmental concerns. In addition,propane that is injected into the formation can be recovered and reused,therefore eliminating the need to treat or dispose of large volumes ofwastewater that may have high concentrations of naturally occurringsalts, metals, radionuclides and other constituents commonly found inshale reservoirs.

Additionally the injection of a hydrocarbon into the shale creates less“damage” due to “swelling” as compared to water, which may impedehydrocarbon flow; therefore LPG fracturing has the potential to increasewell production. While there are a variety of potential benefits tousing LPG fracturing for shale energy development, there may be somepotential disadvantages, such as increased costs for conducting thefracturing treatment, increased explosion hazards, and limited capacityto utilize this technology on a wide commercial basis. One advantagewater-based fracturing technologies have though is that water isvirtually incompressible, therefore the pressure is transferred moredirectly to fracture the shale more effectively, whereas LPG may requiremore surface pressure to exert the necessary downhole pressure.

LPG fracturing is a promising technology that may become more common asadvancements in its use occur and has the potential to reduce water useand increase well yields.

The proposed invention offers safer and a potentially more economicalsolution to conventional and/or energized hydraulic fracturing byutilizing a combination of high-efficiency fracturing fluids (e.g.,water, LPG, etc.) with environmentally safe and inert gases, likenitrogen.

These methods improve natural gas and natural gas liquids recovery fromfraccing operations; reduced flammability of the overall combinationmakes for a safer operation; reduced use of diluents such as carbondioxide or nitrogen allows for better natural gas recovery; costreduction improvements over the use of liquid petroleum gas by itself;and augmentation of the hydrocarbon pressure by using nitrogen or carbondioxide so that high pressure equipment does not need to be employed.

SUMMARY OF THE INVENTION

A method for stimulating an oil or a natural gas well comprising addingto the well a combination of a liquefied hydrocarbon selected from thegroup consisting of liquid petroleum gas and liquefied natural gas, aproppant and a diluent selected from the group consisting of carbondioxide, nitrogen and a mixture of carbon dioxide and nitrogen.

The stimulating of the oil or natural gas well will lead to a fracturingof the fissures in the well thereby leading to an increased yield of oilor natural gas.

The invention may also be employed in fraccing operations of shale gasformations. These methods allow for the recovery of natural gas andnatural gas liquids from frac gas. The well can also be a horizontalwell.

The methods of the invention will provide for the recovery of naturalgas and natural gas liquids from the oil or natural gas well.

In the methods of the invention, the combination is added to the wellsequentially. The liquefied hydrocarbons are typically added firstfollowed by addition of the diluent to the well. Sufficient time istypically allowed for between feeding the liquefied hydrocarbon to thewell before feeding the diluent to the well. The liquefied hydrocarbonsare fed to the well at a first pressure and their pressure is increasedby the addition of the diluent at a higher pressure. In preferredembodiments, the liquefied hydrocarbon is in gel form.

In an alternative embodiment, the diluent is fed to the well before theliquefied hydrocarbon.

Water may be fed to the well along with the liquefied hydrocarbons incertain circumstances. In one embodiment, the water and liquefiedpetroleum gas is first added to the well, followed by natural gas andthen followed by a feed stream of nitrogen.

In a further alternative embodiment, the liquefied hydrocarbon and thediluent are not fed to the well sequentially but are mixed togetherbefore being fed to the well simultaneously.

The liquefied hydrocarbons, whether liquefied petroleum gas or liquefiednatural gas are typically fed to the well at higher pressures in therange of 5000 to 10,000 prig (345 to 690 bar).

The proppant when present is selected from the group consisting ofsilica sand, resin-coated sand and man-made ceramics.

The use of the diluents alters the flammability of the hydrocarboncombination and will allow for easier separation from the well gas thatis derived from drilling operations as described in FIGS. 1, 2 and 3.Carbon dioxide is preferably used because of its higher density, higherheat capacity and easier separation from methane and other hydrocarbons.

Liquefied hydrocarbons can be effectively used in the stimulation of gaswells. Liquid petroleum gas which has higher propane content can beemployed advantageously because of its higher density and highertemperature. Rich liquefied natural gas which has a higher ethane andC3+ content and which is more readily available and economical beingderived from peak shavers will remain a liquid at high pressures even atelevated temperatures as noted in FIG. 4.

The proppants are typically selected from the group consisting of silicasand, resin-coated sand and man-made ceramics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effect of carbon dioxide concentration onmethane flammability.

FIG. 2 is a graph showing the effect of carbon dioxide feedconcentration versus ignition response.

FIG. 3 is a graph showing carbon dioxide feed concentration versus flamepropagation.

FIG. 4 is a chart of vapor liquid equilibriums at various temperaturesand pressures for rich liquefied natural gas.

DETAILED DESCRIPTION OF THE INVENTION

The invention is applicable to horizontal wells utilized inunconventional gas production and consists in the gradual introductionof the fracturing fluid of choice into the well under moderate pressure,followed by introduction of the inert gas under high pressure,sufficient to increase the overall pressure of the fracturing fluid torequired levels.

The fluid introduced first could be water or a liquid hydrocarbon, whichare easy to separate. This step is followed by the addition of a highpressure energized fluid like N₂ and/or CO₂, or natural gas, or evenLPG. It is also possible to have multiple fracturing fluid combinations,for example: water/LPG, followed by natural gas, followed by N₂. The topfluid serves to boost the pressure and ideally to provide anon-flammable blanket. By changing the N₂ and/or CO₂ concentrationprofile in the flowback mixture from exponential decay to a more rapid,near-square shaped decay, this will allow for easier separation of thefracturing fluid from the recovered hydrocarbons. It should be notedthat some diffusion/mixing of the fluids is inevitable, which wouldprevent a user from obtaining a perfect square-shaped drop in N₂ and/orCO₂ concentration profile. The methods of the present invention willyield beneficial results. The invention will improve flowback fluidcleanup. A better recovery of fracturing fluids can be achieved. Theinventive methods are also advantageous in water sensitive formationsthat suffer from water saturation and clay swelling issues. Theinventive methods will work to enhance production of oil inlow-permeability and low porosity well formations as well as shallowformation and those nearly depleted gas reservoirs. Additionally, inthose geographical regions where water shortages are prevalent or thosewith stricter water regulations, the present invention will provide forenhanced gas or oil recovery without using more water as well aslimiting the amount of chemical employed.

The reduced water consumption is beneficial when water is the mainfracturing fluid due to its replacement with inert gas (e.g., nitrogen)in the vertical portion of the well.

The fracturing operation can be safer even when using more risky butmore formation friendly fracturing fluids (e.g., LPG) due to thecompression taking place below the ground under a blanket of a moreenvironmentally safe inert gas such as nitrogen. This may also result inpartial or almost complete elimination of gelling agents, whichotherwise must be added to LPG to reduce the chance of explosion.

The invention further reduces the flowback period and subsequentelimination of the need for natural gas clean-up due to reduced contentof CO₂ and/or N₂ in the flowback mixture.

This latter benefit can be appreciated because the nitrogen for exampleis only present in the portion of the well and is not used in actuallyfracturing the rock formation. Although some mixing and diffusion isunavoidable, the nitrogen is not mixed with the natural gas when used asthe energized solution. This means that once the fracturing is completeand the flowback is started, nitrogen gas will flow out first,relatively quickly and at a relatively constant concentration as opposedto traditional flowback scenario.

The invention is further explained by way of non-limiting examples,provided below.

Using homogeneous energized fluid in the entire wellbore(baseline—longest flowback period).

In this case, a fracturing fluid containing energizing component(s), CO₂and/or N₂, is prepared at the surface and pumped down hole at a selectedpressure that exceeds the reservoir pressure. In the typical fracturingapplication, the first and the last step of the process are done withoutproppant (first is to pump-in the fracturing fluid without the proppant(pad stage) to create fractures, and the last is to clean-up remainingproppant from the wellbore (flush stage). During intermediate stages,the proppant is introduced into the fracturing fluid and its load isgradually increased. This results in quite a lengthy flowback period,required to bring the concentration of the energizing components down tothe required level which is usually 2 to 3%.

Gradual introduction of the fracturing fluid components (shortestflowback period).

In this case, the components of the fracturing fluid are introducedgradually in stages. The first component (water or liquid hydrocarbons)is followed by a high pressure energized fluid like N₂ and/or CO₂serving to boost the pressure and ideally provide a non-flammableblanket. In this case, during flowback, N₂ and/or CO₂ will flow outfirst and their concentration in the flowback mixture will decrease muchmore rapidly as schematically depicted below. It should be noted thatsome diffusion/mixing of the fluids is inevitable, which would preventan operator from obtaining a perfect square-shaped drop in N₂ and/or CO₂concentration profile.

Using a mixture of hydrocarbon(s) and energizing components to pump downhole (intermediate flowback period).

In this case, by using the energizing or diluent component (N₂ and/orCO₂), it becomes possible to safely pump the potentially flammablehydrocarbons (for example, natural gas) down hole at required pressuresdue to diluting them below their flammability limits. Although this willnot result in the shortest flowback period, it does still reduce theconcentration of the energizing component (N₂ and/or CO₂) and therefore,minimizes the cleanup effort.

In general during the practice of the invention, the liquefiedhydrocarbon mixture is pumped up to pressures of about 5000 to 10,000psig before their subsequent use for fraccing. In the methods of theinvention, gelled liquid petroleum gas or gelled liquefied natural gasare the preferred hydrocarbons and can be employed individually or as amixture of the two. Proppants such as silica sand, resin-coated sand andman-made ceramics are added to the hydrocarbon mixture and will act inthe oil or gas well formation to keep fissures open and enhance gasrecovery. A high pressure intermediate gas which is typically carbondioxide or nitrogen can be added either simultaneously with thepressurized hydrocarbon or separately and will serve as a blanket. Theresulting frac mixture becomes less flammable as a result.

Moreover, in a different embodiment, the liquefied hydrocarbon can bepumped at a lower pressure for fraccing and its pressure can besubsequently boosted with the addition of higher pressure carbon dioxideor nitrogen. This pressure boosting by the carbon dioxide and/ornitrogen diluent will provide benefits of both using the liquefiednatural gas or liquid petroleum gas in gel form with the properties ofthe diluent in fraccing operations while minimizing the risks associatedwith the high pressure pumping of hydrocarbons.

Carbon dioxide or nitrogen when used alone in well stimulation andfraccing operations can be effective. However, separation of thesecomponents after a few days time after the well has been drilled becomesmore difficult. For example, carbon dioxide fraccing may yield aninitial gas content that is lean in methane which can only be enrichedvia cleanup once the carbon dioxide content is lowered to 50%. Nitrogenfraccing faces the same limitation. The use of hydrocarbons with eitherof carbon dioxide or nitrogen facilitates natural gas recovery bylimiting the initial amount of carbon dioxide or nitrogen introducedinto the well to a more manageable level.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the present invention.

Having thus described the invention, what we claim is:
 1. A method forstimulating an oil or a natural gas well comprising adding to the well acombination comprising a liquefied hydrocarbon selected from the groupconsisting of liquid petroleum gas and liquefied natural gas, a proppantand a diluent selected from the group consisting of carbon dioxide,nitrogen and a mixture of carbon dioxide and nitrogen.
 2. The method asclaimed in claim 1 wherein the oil or natural gas well is in a shale gasformation.
 3. The method as claimed in claim 1 wherein natural gas andnatural gas liquids are recovered from the oil or natural gas well. 4.The method as claimed in claim 1 wherein the well is a horizontal well.5. The method as claimed in claim 1 wherein the combination is added tothe well sequentially.
 6. The method as claimed in claim 1 wherein theliquefied hydrocarbon is added to the well first, followed by thediluent.
 7. The method as claimed in claim 1 further comprising addingwater to the well along with the liquefied hydrocarbon.
 8. The method asclaimed in claim 1 wherein the water and liquefied petroleum gas isfirst added to the well, followed by natural gas, followed by nitrogen.9. The method as claimed in claim 1 wherein sufficient time is allowedfor between feeding the liquefied hydrocarbon to the well before feedingthe diluent to the well.
 10. The method as claimed in claim 1 whereinthe liquefied hydrocarbon and the diluent are fed to the well together.11. The method as claimed in claim 1 wherein the diluent is added to thewell before the liquefied hydrocarbon.
 12. The method as claimed inclaim 1 wherein the liquefied hydrocarbon is fed to the well at apressure of 5000 to 10,000 psig.
 13. The method as claimed in claim 1wherein the liquefied hydrocarbons are fed to the well at a firstpressure and increased in pressure by the addition of a higher pressurediluent.
 14. The method as claimed in claim 1 wherein the liquefiedhydrocarbon is in gel form.
 15. The method as claimed in claim 1 whereinthe proppant is selected from the group consisting of silica sand,resin-coated sand and man-made ceramics.